1. Field of the Invention
The present invention relates to a tire condition monitoring system for a commercial vehicle having a plurality of wheels and an installation position ascertaining method for a tire condition measuring device.
2. Background Information
In the field of tire condition monitoring systems for passenger cars, there is a known technology contrived to automatically detect the positions of four wheels using a rotation direction of a tire and a received signal strength (RSSI: received signal strength indication) of a radio signal. For example, PCT Application No. 2002-531319 discloses a method of allocating identifiers contained in a plurality of signals issued from a plurality of transmitters. In order to accomplish this goal, a tire air pressure as well as an acceleration value calculated based on a wheel movement state are measured for each of the monitored wheels. In order to identify a plurality wheels on a right side of a vehicle and a plurality of wheels on a left side of a vehicle, a sine wave of an acceleration along a path B is determined and a sine wave of a change of a centrifugal acceleration Z occurring during acceleration is determined for each individual wheel. Then, products of the two sine waves determined for different wheels are compared to identify a plurality wheels on a right side of a vehicle and a plurality of wheels on a left side of a vehicle.
Additionally, signals obtained from the front wheels and signals obtained from the rear wheels are identified by comparing the strengths of signals received at each receiving antenna among a plurality of receiving antennas. A receiving antenna arranged at the wheels is monitored and signals having a higher strength are assigned to front wheels while signals having a lower strength are assigned to the rear wheels. Meanwhile, receiving antennas arranged at the rear wheels are monitored and signals having a higher strength are assigned to the rear wheels while signals having a lower strength are assigned to the front wheels. In this way, a position of a sensor is ascertained by identifying whether the sensor is located on a left wheel or a right wheel based on a phase of a sine wave of an acceleration signal and estimating whether the sensor is positioned on a front wheel or a rear wheel based on the strengths of the radio signals.
In addition, Laid-open Japanese Patent Application Publication No. 2008-74223 discloses a wheel position estimating method in which a wheel information transmitting device comprises an air pressure sensor contrived to detect wheel information, a control circuit contrived to store identification information for distinguishing the wheel on which the wheel information transmitting device is installed from other wheels, and a wheel communication device contrived to transmit identification information for the wheel on which the wheel information transmitting device is installed as wheel information and to receive identification information for other wheels from other wheel communication devices. The control circuit is contrived to compare the strengths of received signals containing identification information for other wheels to a threshold value and calculate position information indicating whether or not the wheel on which the wheel information transmitting device is installed is one wheel of a dual wheel. The wheel information transmitting device is also contrived to calculate position information indicating relative positions of the wheel on which the wheel transmitting device is installed with respect to the other wheels based on the strengths of the received signals containing identification information for the other wheels and to transmit identification for the wheel on which the wheel transmitting device is installed, identification information for the other wheels, and the aforementioned position information at a prescribed timing. In this way, a position of a dual wheel is determined using signal strengths.
Also, Japanese Translation of PCT Application 2010-525990 discloses a position determining method for determining positions of sensors on wheels (200, 300) that are installed coaxially on the same end portion of the same wheel axle of a vehicle (C) equipped with a receiving module (100) joined to a chassis of the vehicle (C). Positions of the sensors (210 and 310) are determined based on an angular spacing of 90 degrees or larger between one wheel (200) and the other wheel (300). As a result, when the vehicle (C) moves, the distances between the receiving module and the sensors change as the wheels rotate. Since a distance exists between signal emission points, an electric power of the received signals can be made to fluctuate by using a low frequency (LF) connection. Thus, the received signals can be differentiated. In other words, the positions of dual wheels are determined using the low frequency (LF) connection and the radio signal strength of the sensors as the sensors passing through a vicinity of the receiver.
Furthermore, Laid-open Japanese Patent Application Publication No. 2008-143490 discloses a system comprising a plurality of wheel condition detecting units provided to correspond to each of a plurality of wheels and contrived to transmit wheel condition information indicating a detected condition of a wheel to an external location wirelessly, a receiver that is provided on a main body of a vehicle onto which the aforementioned wheels are installed and contrived to receive the aforementioned wheel condition information, a plurality of motors each of which is provided adjacent to one of the aforementioned wheels and contrived to drive the wheel, a drive control means contrived to control operation of the motors, and a detection target wheel ascertaining means contrived to ascertain which wheel is targeted for detection by each of the wheel condition detecting units. The system determines which wheel is targeted for detection by each of the wheel condition detecting units by having the drive control means operate a prescribed motor among the plurality of motors and having the detection target wheel ascertaining means determine which wheel condition detecting unit corresponds to the wheel adjacent to the motor that was operated. More specifically, the detection target wheel ascertaining means uses wheel condition information received from the wheel condition detecting units to ascertain which of the wheel condition detecting units corresponds to the wheel whose adjacent motor operated at the time when the aforementioned prescribed motor was operated. In this way, when an in-wheel motor is operated, the wheel can be specified based on a corresponding temperature change and noise.